Recently, optical elements utilizing an optical diffraction phenomenon are utilized in various fields. FIG. 10 shows a general diffraction optical element 122. The diffraction optical element 122 is formed by forming parallel grooves having a triangular wave cross-section in a flat plate. When light enters the diffraction optical element 122, the light is split into diffracted beams of the zero order, the first order, the second order, etc. A method of evaluating diffraction efficiency of a beam diffracted in each order will be described below in detail.
FIG. 11 is a diagram explaining the diffraction efficiency evaluation method called a knife-edge method. When laser light 121 enters the diffraction optical element 122, it is split into diffracted beams having different orders which are focused on corresponding points in the direction perpendicular to the optical axis. The image-forming spot beams are blocked and scanned at predetermined intervals with the use of a mask plate 123 such as a knife-like edge sufficiently longer and thinner than the spot diameter so that the intensity of a spot beam at each point through which the beam passes is measured by a photodetector 124. From the obtained intensity distribution, the intensity difference is calculated, and the diffraction efficiency in each order is calculated from the thus obtained difference.
Patent Document 1 discloses a diffraction efficiency measurement of a diffraction optical element with cocentric diffraction grooves configured to form image forming points of spot beams diffracted in orders on the same straight line. In this measurement method, laser light is paralleled by a collimator lens and is allowed to enter a diffraction optical element, and then, the beam converged by the diffraction optical element is magnified by and observed under a microscope. In so doing, the spot beams of the orders overlap with each other. However, a pinhole slit is disposed around the diffracted spot beams for eliminating unnecessary order beams to remove the influence thereof. Computation of the diffraction efficiency is carried out in such a manner that the amount of transmitted light is first measured with the diffraction optical element not set as an incident light amount, and then, each amount of the spot beams diffracted in the orders transmitted through the pinhole slit is obtained.
However, the above two methods involves the following problems.
The knife-edge method as a general diffraction efficiency evaluation method is applicable only to the case where a plurality of diffracted spot beams are formed alongside perpendicularly to the optical axis. The diffraction optical element with cocentric diffraction grooves cannot be evaluated by the knife-edge method because a plurality of diffracted spot beams are formed side by side along the optical axis.
Patent Document 1, which evaluates the diffraction optical element with cocentric grooves, uses laser light as a light source for evaluation, and is addressed to an optical pickup lens. Therefore, only the diffraction efficiency in a single wavelength can be evaluated. Further, Patent Document 1 premises laser light providing a sufficient light amount as the light source. Therefore, a light source having wavelengths ranging to some extent, which has less light amount than the laser light, cannot be measured by the method in Patent Document 1.
In addition, the method in Patent Document 1 uses the pinhole slit for removing the influence of the sport beams in unnecessary orders, which means that slits with various hole diameters must be prepared for addressing the spot diameters of the spot beams diffracted in various orders. It is ideal for accurate evaluation to make the slit sizes agree with the spot sizes. This imposes a limit on the evaluation using the pinhole slit. In the second embodiment in Patent Document 1, the amount of light passing through an aspheric lens, rather than a diffraction optical element, is obtained in advance as an incident light amount for obtaining the diffraction efficiency. However, this method requires to prepare an additional lens having the same effective diameter as that of the diffraction optical element, which is laborious.    Patent Document 1: Japanese Unexamined Patent Application Publication 09-196813